1. Field of the Invention
The present invention relates to cell search methods and mobile station apparatuses used for cellular systems such as digital car telephones and portable telephones.
2. Description of the Related Art
In cellular radio systems such as car telephones and portable telephones, the FDMA (Frequency Division Multiple Access) system and TDMA (Time Division Multiple Access) system, etc. are conventionally known as multiple access systems in which multiple stations perform communications simultaneously using the same frequency band.
In addition, the CDMA (Code Division Multiple Access) system using a spread spectrum system is a system which achieves higher frequency utilization efficiency and accommodates more users than these techniques.
This spread spectrum refers to a system which transmits a signal which contains information by spreading it to a band wider than the band of the signal. It is divided into two types; a direct sequence (DS) system and frequency hopping (FS) system. Of these two systems, the direct sequence/spread spectrum (DS/SS) system is the mainstream for systems introduced for satellite communications and ground mobile communications because it has the advantage that it can easily be implemented in actual apparatuses.
This spread spectrum communication system is a system in which information signals are spread over a sufficiently wide range of radio bands compared to a minimum required bandwidth, providing excellent confidentiality, secrecy and interference resistance. On the other hand, the direct sequence system is a spread spectrum communication system in which a spreading code is carried on an information signal as it is when it is spread.
In such a CDMA system, it is effective to use codes with a high level of orthogonality as spreading codes for the purpose of increasing the capacity. However, since the number of Walsh codes or orthogonal Gold codes known as codes with a high level of orthogonality is limited to the same number as their code length, securing the number of spreading codes to be assigned to users requires the use of a combination of short codes whose cycle is equal to the symbol length of information and long codes whose cycle is greater than that of these short codes (USP5103459).
In this case, using a method of using one long code for all base station apparatuses in the down link and assigning different long code phases to different base station apparatuses can maintain the orthogonality of all users in the same cell. Furthermore, since signals in other cells are spread with different long code phases, they are converted to noise, making it possible to minimize interference. In a system using such a long code, a mobile station needs to acquire and maintain synchronization of the long code during communication.
In a cellular system, if a mobile station is powered on, or if communication between the mobile station and base station apparatuses is interrupted, or when the mobile station carries out handover, that is, it switches the base station apparatus with which it communicates as the position of the mobile station changes during communication, if the mobile station specifies the switched base station apparatus, the mobile station needs to identify which base station apparatus is currently closest to it and which base station apparatus is the best one to communicate with. This is called "cellular search."
In a cellular system using the CDMA system, one of the cellular search methods is a method in which all base station apparatuses transmit a pilot channel which is spread with the same long code, while a mobile station apparatus carries out correlative detection of all phases of the spreading code of the pilot channel received, and the base station apparatus which is transmitting with the phase having the maximum correlative value, that is, the maximum strength of the pilot channel is identified as the base station apparatus closest to the mobile station apparatus (USP4901307).
Furthermore, there is another method to achieve a speedy cell search: When a mobile station apparatus carries out a cell search while communicating or waiting for communication, the base station apparatus with which it is currently communicating notifies it of the range of the phase presence (called "search window width") estimated from the long code phase information used by base station apparatuses which currently exist in the periphery, differences in the reception timing of the mobile station apparatus due to differences in locations where the peripheral base station apparatuses are installed, and multi-path delay variance, thus limiting the long code phase to be searched by the mobile station apparatus, and hence speeding up a cell search.
In addition, there is a 2-stage pause synchronization capturing method as the method to carry out a high-speed cell search. In the 1st stage of this method, correlative detection is carried out on phases in the search window sequentially with an integrating length which is set every time shorter and if the correlative value exceeds a threshold, the correlative value is obtained with an integrating length long enough to achieve the accuracy of the correlative value, and the strength of the pilot channel is detected in this way.
However, if the mobile station apparatus is moving at a high speed, or if base station apparatuses are clustered close together in big cities, etc. where there is a great number of subscribers, the number of phases to be searched increases and the surrounding cell environment changes quickly, and thus this method has the problem that it requires faster cell search than the conventional technology above. Furthermore, in the 2-stage pause synchronization capturing method, if a threshold to be set is too low, almost all phases need long-hour integration, making it difficult to carry out a high-speed cell search. On the other hand, if a threshold to be set is too high, there is no phase that exceeds this threshold, making a search itself impossible. In particular, this method has the problem that it is difficult to set an appropriate threshold in an ever-changing mobile communication environment.